This invention relates to an apparatus for inductively generating plasma. It relates specifically to a robust and low-cost apparatus for producing a compact volume of high-density plasma. More broadly, this invention relates to methods for performing a variety of useful industrial process such as generating reactive gasses, processing semiconductors, destroying gaseous toxic waste, forming nano-particles, and enhancing gaseous chemical processes using the novel apparatus described herein.
Gaseous plasma discharges are widely applied in numerous industrial and technological processes. In particular, plasmas are used in many semiconductor manufacturing processes, as well as welding, plasma spraying of materials, nano-particle generation and ion sources. In addition to thermal processes like plasma-spraying and welding, a plasma is an efficient means of enhancing chemical reactions. A plasma will break apart the molecules of a feed gas, producing a highly reactive mixture consisting of the incoming feed gas plus neutral radicals, ions, atoms, electrons, and excited molecules. The plasma is therefore widely useful as a ‘chemical factory’ capable of cracking molecules into lower order forms, breaking down molecules into their atomic constituents, and promoting volume- and surface-based chemical reactions with other molecules that would not otherwise occur.
The many different means of plasma generation known in the art fall into four broad categories depending on how energy is coupled into the plasma. These consist of:
a) DC excitation, in which at least two electrodes are in direct contact with the plasma. Electrical current is made to flow from one electrode to another, through the plasma, thereby transferring energy to the plasma.
b) Capacitive excitation, in which an alternating voltage across two separate electrodes produces an alternating electric field between the electrodes that causes AC current to flow through the plasma. This method is similar to DC excitation, except that the electrodes need not be in direct contact with the plasma, since power is coupled into the plasma capacitively across the plasma sheath.
c) Inductive excitation, in which alternating current is passed through coil located near the plasma. The coil produces an alternating magnetic flux in the plasma. This alternating magnetic flux induces current to flow inside the plasma, according to Faraday's law of electromagnetic induction, thereby heating the plasma. Inductively excited plasmas are often referred to as “inductively-coupled” or equivalently “transformer-coupled” plasmas, since the coil functions electrically as the primary winding of a transformer and the plasma itself plays the role of the secondary winding of the transformer; the two windings being electrically coupled together by AC magnetic flux.
d) Resonant excitation. This category includes a wide variety of excitation methods that transfer energy into the plasma by exciting waves or natural resonances of the plasma. These methods include most commonly microwave and helicon excitation.
The method of DC excitation is often employed in high-pressure thermal arc plasmas that are primarily used in the heating of materials; for example welding and plasma spraying. DC glow discharges, which typically operate at lower pressures, are frequently used in cleaning metallic surfaces. In either case, the DC discharge generally is accompanied by the erosion of one of the electrodes due to thermal or sputtering effects. Although erosion is desired for some applications such as welding, in many fine processes, such as semiconductor processing, electrode erosion represents a source of metals contamination and is highly undesirable.
Capacitive plasma excitation has been widely applied in the manufacturing of semiconductor chips. In contrast to the DC discharge, it is possible to protect the electrodes of a capacitively excited plasma with a dielectric covering that reduces metals contamination, yet still permits power to be delivered into the plasma. Nevertheless, to achieve significant capacitive power transfer to the plasma it is necessary to drive the electrodes to relatively high voltages. These voltages are often in the hundreds or even thousands of volts. Thus, the mean plasma potential relative to a grounded chamber will be rather high, as will the instantaneous potential between the plasma and the electrodes. These potentials appear across the plasma sheath. Positive ions that reach the plasma boundary will subsequently be accelerated through the sheath toward the chamber walls and the powered electrodes and will reach energies corresponding to the potential that appears across the sheath. Consequently, these ions can be accelerated to energies that are sufficient to sputter electrode and chamber material into the plasma. Not only can this produce plasma contamination and a gradual erosion of the chamber walls, but it also represents a significant source of power loss for the plasma. High plasma potentials and high sheath voltages are undesirable.
More recently, the trend in semiconductor processing has been toward the use of inductively excited plasma. This is primarily because inductive plasmas have higher densities and lower voltages. It is known among those skilled in the art that inductive excitation is a more efficient means of heating a plasma. Inductive plasmas are characterized by substantially higher plasma densities and therefore result in correspondingly faster, more productive processing methods. Inductive plasmas also tend to have significantly lower plasma potentials and sheath voltages, which significantly reduces the problems associated with capacitive excitation described above.
Hittorf made the first inductively heated plasma in 1884. In the classic configuration, a cylindrical tube made of glass, quartz, ceramic, or other dielectric is wrapped with a coil comprising a number of turns. A working gas at some controlled pressure is sealed inside the tube or caused to flow through it. The ends of the coil are connected to a source of AC power, which drives an alternating current through the coil. This AC coil current in turn establishes an alternating longitudinal magnetic field inside the tube that induces current to circulate through the conductive plasma. The induced plasma current circulates around the axial magnetic flux in a direction opposite the applied coil current, according to Faraday's law.
Even today, this simple design is applied quite widely. At high pressures in the working gas, this configuration is commonly referred to as an inductively-coupled plasma torch. At lower pressures, this cylindrical design is often used in semiconductor processing equipment. Another variation of the inductively-coupled plasma uses a flat, spiral-shaped coil coupled to the plasma through a flat dielectric window. This “electric stovetop” coil design generates a uniform plasma over a large area, and thus has proven to be well suited for processing the large flat substrates such as the silicon wafers used in microchip manufacturing.
Finally, resonant plasma excitation is known to be effective at producing plasmas of very high density and low sheath voltages. Microwave plasmas in particular, are now widely used in semiconductor processing equipment. Generally, a resonantly excited plasma must be immersed in a precisely controlled DC magnetic field. The overall cost, complexity and size of such a system is relatively large compared to an inductive system, due to the microwave power supply, a microwave tuner, DC magnetic field coils and their associated DC power supplies. These drawbacks often preclude the use of resonant excitation in many applications.
The use of inductively heated plasma appears to be generally advantageous for many industrial applications. It is simpler and less costly than resonant excitation, yet it is superior to DC and capacitive excitation because of high plasma density and low sheath voltage. On the other hand, inductive plasmas do have some weaknesses toward which this invention is directed.
First, although the problem of erosion and contamination caused by the high voltage sheath is reduced when compared to a capacitive or DC discharge, it is not completely eliminated. Recall that in an inductive plasma, the coil, of N turns, forms the primary of a transformer and the current loop, inside the plasma itself, forms the one-turn secondary of the transformer. (This transformer will henceforth be referred to as the plasma transformer in order to distinguish it from the matching transformer, to be introduced later). Higher plasma currents result in higher plasma densities, therefore, based on the well known electrical behavior of transformers, it seems advantageous to increase the number of primary turns, N. Unfortunately, this strategy leads to higher voltages across the primary coil of the plasma transformer. These high voltages, especially near the ends of the primary coil, couple capacitively to the plasma and produce high energy ion bombardment of the walls resulting in sputter contamination, wall erosion, and energy loss in these areas.
One well-known means of addressing this problem has been to employ an electrostatic shield between the coil and the plasma. Such shields are designed to be electrically conductive in the direction of the electric field that appears end-to-end across the terminals of the coil, but electrically non-conductive in the direction of current flow. In this way, the coil's electric field is shunted away from the plasma, while the magnetic flux is not. The shields typically comprise a series of metal strips running perpendicular to the direction of current flow. In practice, however, the oscillating magnetic flux induces eddy currents in the shield, thereby absorbing part of the applied power.
Another problem with inductive heating is the need for a tube, chamber wall, or window made of dielectric material. Materials such as ceramic, quartz, or glass are typically used. Since plasma processes are often operated at low pressure, these parts must be strong enough to withstand external atmospheric pressures, often over large areas. They must also be able to efficiently transmit the flux of primary coil into the plasma volume. Finally, they must withstand the temperatures and thermal stresses resulting from heat flowing out of the plasma to the walls of the plasma chamber.
Ceramics and glasses are brittle materials that are sensitive to thermal shock or slight mechanical imperfections. They can shatter explosively under vacuum pressure. Many applications of plasmas also involve the processing of toxic gasses, particularly in semiconductor manufacturing and gaseous waste treatment. The use of these brittle chamber materials with toxic gasses poses a risk of sudden uncontrolled release. Furthermore, heat deposited on the inside surface of the plasma chamber must somehow be removed. Unfortunately, most dielectric materials have poor thermal conductivity. The difficulty of cooling the dielectric portion of the plasma chamber is compounded in large volume applications by the need to make the chamber wall thick enough to withstand vacuum pressure. Finally, these dielectric materials are costly. The cost grows very rapidly as the dimensions of the chamber are increased. For all these reasons it would be advantageous to find an alternative to the large areas of dielectric chamber material.
Another weakness of most inductively coupled plasma reactors of cylindrical or planar coil geometry is related to their topology. Magnetic field lines always form closed curves. For example, in the cylindrical geometry of the inductively-coupled plasma torch, the primary coil produces a dipole magnetic field: the field passes through the center of the coil on the inside of the plasma chamber. At the ends of the coil, however, the field inevitably penetrates through the chamber wall and closes upon itself on the outside of the coil. This external magnetic flux is in a sense ‘wasted’ since it does not contribute to the heating of the plasma. Furthermore, were the plasma chamber to made of conductive material such as metal, the magnetic flux penetrating through the chamber wall at the coil ends would induce eddy currents in the chamber wall, resulting in significant power loss and inefficient heating of the plasma. Even in a chamber made of dielectric material, the magnetic field extends a significant distance outside the chamber. This stray field can produce severe electromagnetic interference for nearby equipment and, depending on the frequency, can illegally interfere with radio communications. The interference is generally suppressed with a metal enclosure or shielding around the plasma reactor, but the stray field will induce eddy-currents in the shielding, resulting in power loss. In summary, there are undesirable eddy-currents induced in metal surfaces wherever the magnetic field created by the primary coil penetrates a metal surface.
The topology of the torus has long been recognized among designers of nuclear fusion equipment as particularly desirable. The fundamental reason is that a toroidal surface can be described by two cyclic, or closed, dimensions that are orthogonal to each other. Since magnetic fluxes and the associated AC electrical currents always form closed loops, and are orthogonal to each other, the torus lends itself to plasma reactor design.
Excluding nuclear fusion reactors, the toroidal design is not commonly applied in industrial plasma reactors. Nevertheless, an early reference to an inductively-coupled toroidal plasma can be found in IEEE Transactions on Plasma Science, Vol. PS-2, 1974 by H. U. Eckert. U.S. Pat. No. 4,431,898 teaches the use of an inductively coupled toroidal reactor for semiconductor manufacturing. Similar teaching is found in Japan patent 02-260399, and U.S. Pat. No. 5,290,382. Recently, U.S. Pat. No. 6,150,628 described a toroidal reactor having a metal chamber. All of this prior art is fundamentally similar, comprising:
a) a toroidal plasma chamber;
b) a closed magnetic ring of ferrite or laminated iron passing through the center hole of the toroidal plasma chamber and closing around it;
c) a wire, forming the transformer primary winding, wrapped around the magnetic ring such that the turns pass through the center hole of the magnetic ring;
d) an AC power source coupled to the ends of the primary winding.
In this way, the primary winding generates an AC magnetic flux that is confined to the magnetic circuit formed by the ring of magnetic material. The AC magnetic flux, passing through the center of the plasma induces currents in the plasma that circulate around the flux and, therefore, around the center hole in the plasma chamber. The essential feature is that the plasma forms a closed loop surrounding the flux-carrying magnetic core.
This design suffers from the large quantity of magnetic material required. Because the magnetic material must entirely surround the plasma itself, as well as the plasma chamber, a rather large amount is needed. At low frequencies such as 60 Hz, one may use a laminated iron core, which is inexpensive, but heavy and very bulky. At higher frequencies, where it is more desirable to operate most inductive plasmas, expensive ferrite materials are required. The long magnetic circuit also tends to limit the efficiency of power transfer through the transformer. At the frequencies above 10 MHz, where most semiconductor processing plasmas operate, ferrite materials become rapidly more lossy and more expensive.